I need an driver installer for this computer

There is a folder on Drive C called "DellDrivers"

Otherwise, all your drivers are on Dell's website:
http://support.dell.com/support/downloads/driverslist.aspx?c=us&cs=19&l=en&s=dhs&ServiceTag=&SystemID=PLX_PNT_CEL_GX60&os=WW1&osl=en&catid=&impid=

Windows has the Installation Wizard. This is the driver installer. After you download a driver, open it. (Double-click on the driver file name)

Installation Wizard will come up, and guide you through installation. When the Installation Wizard comes to the part, where it tells you where it's going to install the file. Write it down if you wish, but Don't change it. It will be installed to the correct place.
(Therefore click on Next, Next, and Finish)

Here is Dell Support, and the Drivers and Downloads page for the Dell Optiplex GX260,
(I believe the heading above stating GX60 is erroneous. Should be GX260)

http://support.dell.com/support/downloads/driverslist.aspx?c=us&l=en&s=gen&ServiceTag=&SystemID=PLX_PNT_P4_GX260&os=WW1&osl=en&catid=&impid=

If you have a specific driver in mind, and do not know which one you need, come back, and tell me.

If you are unclear about the procedure to download drivers, and install them, come back and tell me.

Old machine ...

go to Dell.com/support and look around for your model ... get the software you need